Internal combustion engines are typically run by liquid fuel that is contained within a fuel tank. An air space exists above the surface of the liquid fuel within the fuel tank. Over time, this air space becomes filled with evaporative emissions (i.e., fuel vapor) from the liquid fuel and can be under pressure. It is desired that the amount of such evaporative emissions contained within the tank be minimized for multiple reasons, the primary reason of which is to minimize the emission of hydrocarbons into the atmosphere.
A typical fuel tank assembly generally comprises of a reservoir portion, a neck portion, and a fuel cap that seals the neck portion. The neck portion of the fuel tank is typically integrally formed as a single unit with the reservoir portion. When the level of the liquid fuel in the reservoir portion is low, the vapor region contains a large amount of fuel vapor under pressure. Consequently, when the fuel cap is detached from the neck portion, the fuel vapor is forced out of the fuel reservoir and into the outside air, causing air pollution.
One approach used by the industry to reduce these evaporative fuel emissions is to incorporate a filtration system into the fuel cap that vents the vapor region and adsorbs hydrocarbons from the vented fuel vapor prior to being released into the atmosphere. The hydrocarbon filtration is typically accomplished by venting the fuel vapor through a chamber within the fuel cap containing activated charcoal. The activated charcoal has a natural affinity for hydrocarbons when in direct communication with fuel vapor. Once the activated carbon becomes saturated with hydrocarbons it must be purged with fresh air to unload the carbon particles of these hydrocarbons. The purging of hydrocarbons with fresh air can be achieved using two methods: (1) an “active purging” method that uses a pumping source to draw fresh air through the carbon containing chamber; and (2) a “passive purging” method that uses naturally occurring conditions to create air flow through the carbon containing chamber. For example, passive purging can be achieved by the small vacuum that is inherently created within the fuel tank reservoir as the fuel level drops, thereby drawing fresh air from the atmosphere through the carbon containing chamber. Passive purging can also occur due to changes in temperature of the fuel and/or fuel vapor, even when the engine is not running.
Fuel caps utilizing a passive purge are known in the art and have become desirable due to their simplicity of manufacture and case of use. However, existing passive purge fuel caps suffer from a number of drawbacks, including: inadequate purging of the hydrocarbon adsorbing media; inadequate fuel vapor flow through the hydrocarbon adsorbing media; and a tendency to become contaminated by splashed liquid fuel from the reservoir. Thus, a need exists for an improved passive purge fuel cap.